The present invention relates to novel concentrated leuco indigo solutions consisting essentially of from 25 to 55% by weight of leuco indigo in the form of the leuco indigo salt, an amount which is essentially stoichiometrically sufficient to convert the leuco indigo into the neutral leuco indigo salt of a mixture of at least two alkali metal hydroxides wherein none of the alkali metal hydroxides accounts for more than 70 mol %, and water.
This invention further relates to the preparation of these leuco indigo solutions and to their use for dyeing cellulosic textile material.
Indigo is a well-known vat dye which is used for dyeing cellulosic textile material, especially cotton warp yarns for blue denim articles.
To dye the water-insoluble indigo, it first has to be reduced (vatted) to convert it into the water-soluble leuco form which has affinity for fiber and which, after going on to the material to be dyed, is then oxidized back to the water-insoluble vat dye pigment.
WO-A-94/23114 discloses an ecologically advantageous dyeing process which utilizes indigo in the form of the caustic aqueous leuco indigo solution obtained by catalytic reduction of an indigo suspension and which dramatically reduces the levels of sulfate or organics in the wastewater compared with the conventional dyeing processes.
Since, however, it has hitherto not been possible to prepare concentrated leuco indigo solutions which are stable and do not tend to crystallize out, the leuco indigo solutions used in WO-A-94/23114 have only a leuco indigo content of 20% by weight, too.
It is an object of the present invention to provide highly concentrated leuco indigo solutions which permit a further reduction in the dispensing volumes needed for dyeing and hence are even more effective in reducing the risk of dyebath overflow and also make a contribution to lowering the costs of transporting the dye solution from the manufacturer to the dyer.
We have found that this object is achieved by the concentrated leuco indigo solutions defined at the beginning.
This invention further provides a process for preparing these leuco indigo solutions by catalytic hydrogenation of alkaline aqueous indigo suspensions, which comprises performing the hydrogenation in stages and first hydrogenating a from 20 to 30% strength by weight indigo suspension comprising from 4 to 10% by weight of a mixture of at least two alkali metal hydroxides wherein none of the alkali metal hydroxides accounts for more than 70 mol % to form leuco indigo, then adding further indigo to set the ultimately desired leuco indigo content and also, if appropriate, alkali metal hydroxide mixture, and continuing the hydrogenation.
The present invention also provides a further preparation process which comprises first catalytically hydrogenating a from 20 to 30% strength by weight alkaline aqueous indigo suspension comprising from 4 to 10% by weight of the alkali metal hydroxide mixture described above and setting the resulting leuco indigo solution to the desired leuco indigo content by evaporation.
The present invention lastly provides for the use of the leuco indigo solutions for dyeing cellulosic textile material.
The leuco indigo solutions of the present invention comprise from 25 to 55% by weight, preferably from 30 to 55% by weight, particularly preferably from 35 to 50% by weight, very particularly preferably from 40 to 45% by weight, of leuco indigo in the form of leuco indigo alkali metal salt.
The second essential constituent of the leuco indigo solutions of the present invention is the alkali metal hydroxide mixture with which the leuco indigo vat acid is converted into the leuco indigo vat salt.
The alkali metal hydroxide mixture of the present invention comprises at least two different alkali metal hydroxides subject to the proviso that none of the alkali metal hydroxides should account for more than 70 mol % in the mixture.
Suitable alkali metal hydroxides include for example lithium hydroxide, sodium hydroxide and potassium hydroxide, which may all be present as a triple mixture or in the form of the respective double combinations, in which case mixtures of sodium hydroxide and potassium hydroxide are preferred.
In mixtures of two alkali metal hydroxides, the proportion of one of the hydroxides is preferably within the range from 70 to 30 mol % and that of the other correspondingly within the range from 30 to 70 mol %. Particular preference is given to levels of from 60 to 40 mol % on the one hand and from 40 to 60 mol % on the other. Very particularly preferably, the two hydroxides (especially sodium hydroxide and potassium hydroxide) are present in approximately equal weight quantities, i.e., their weight ratio is within the range from 0.8:1 to 1.2:1, especially about 1:1.
The leuco indigo solutions of the present invention include the alkali metal hydroxide mixture in an amount which is essentially stoichiometrically sufficient for the complete formation of the neutral vat salt, i.e., generally from 1.5 to 2.5 mol of alkali metal hydroxide mixture per mole of leuco indigo.
Surprisingly, the leuco indigo solutions of the present invention are very stable and, in contradistinction to the known leuco indigo solutions in which the leuco indigo is present as a pure sodium salt, do not tend to crystallize out. For instance, the leuco indigo solutions of the present invention of up to about 40% by weight have an unlimited storage life under nitrogen at low temperatures down to about 10xc2x0 C. (30-35% strength by weight solutions can even be transported and handled when it is frosty); solutions from 50 to 55% strength by weight are better stored at from about 40 to 60xc2x0 C. to avoid crystallization.
The leuco indigo solutions of the present invention can advantageously be prepared by the two processes likewise disclosed in the present invention.
In one of the processes, the preparation is effected by stagewise catalytic hydrogenation. The procedure employed is advantageously as follows:
First, a low concentration alkaline aqueous indigo suspension, comprising about 20-30% by weight of indigo and 4-10% by weight of the alkali metal hydroxide mixture of the present invention, is hydrogenated in a conventional manner using a nickel catalyst (Raney nickel) at a hydrogen pressure of typically 2-6 bar and a temperature which is generally within the range from 60 to 90xc2x0 C. This is followed by the addition of the amount of indigo missing to set the ultimately desired leuco indigo content, either as solid or as aqueous suspension, and also, if insufficient alkali metal hydroxide was used in the first step, of a supplementary amount of alkali metal hydroxide mixture, and the continuation of the hydrogenation. After the catalyst has been filtered off, the leuco indigo solution is stored under a protective gas such as nitrogen.
In general, the two hydrogenation steps together take from about 10 to 12 h.
In the other process, an about 20-30% strength by weight leuco indigo solution is initially prepared, by catalytic hydrogenation, similarly to the above-described process and is then concentrated by evaporation until the desired leuco indigo content is obtained. The distillative removal of the water preferably takes place in the absence of oxygen at from about 60 to 1000 mbar and from 45 to 100xc2x0 C.
Either process of the invention provides a simple reproducible way of preparing the novel leuco indigo solutions.
They are advantageously useful for dyeing cellulosic textile material, for which the procedure described in WO-A-94/23114 may be employed.